- "One of their light walkers carried a weapon of lethal effect. It fired a form of ultra-high velocity projectile. I saw one of our tanks after having been hit by it. There was a small hole punched in either flank - one the projectile's entry point, the other its exit. The tiny munition had passed through the vehicle with such speed that everything within the hull not welded down had been sucked out through the exit hole. Including the crew. We never identified their bodies, for all that remained of them was a red stain upon the ground extending some twenty metres from the wreck."
- — Major Kane, Mordian 607th Heavy Armoured Regiment
A Railgun is a heavy kinetic weapon that makes use of electromagnetically-induced acceleration to fire a projectile at high speeds. The Railgun is commonly mounted on several potent Tau Fire Caste units, including the Hammerhead Gunship and the XV88 Broadside Battlesuit, though larger variants have been employed as starship weapons by the Kor'vattra (Tau Navy).
A Railgun uses a series of powerful electromagnets to accelerate small, solid and conductive projectiles to speeds close to a significant fraction of the speed of light. The resulting high levels of kinetic force imparted to the weapon's projectile are then capable of destroying even heavily armoured vehicles. Few vehicles are able to withstand a direct hit from a Railgun without suffering critical damage. Because the projectile is capable of achieving a velocity of anywhere between six and ten times the speed of sound, it generates a tremendous force when it strikes its target that is often far more than would be produced by an explosive charge of the same size. As such, the speed of the projectile is enough to punch through virtually any amount of armour. The size of the weapon determines the size of its power source and thus the kinetic force imparted to the type of projectile used as ammunition. Railguns usually fire single, solid projectiles and rely on the projectile's speed to penetrate armour, even at long ranges. Because the projectiles are so small, large numbers can be carried by the weapon's operator at a time. However, when used against infantry, a Railgun can have a devastating effect where the round will simply vaporise the target. Those near the travelling path of the projectile will also find the breath sucked out of their lungs, such is its speed. A Railgun makes an iconic "whip-crack" sound when it fires,which is created when the Railgun's round breaks the sound barrier.
The basic physical principle behind Railgun technology is well known to the Adeptus Mechanicus, but the Imperium has never been able to utilise it in a worthwhile form due to the myriad of issues inherent in the operation of such weapons. The limitations of such technology include the staggering amount of power required to propel the projectile along the rail, and the dissipation of the heat generated by the process. The Tau appear to have implemented solutions to these problems, and many Hereteks amongst the Adeptus Mechanicus would very much like to understand how.
A Railgun can be used in conjunction with a Marker Light to make it even more accurate. Railguns also exist in a smaller and far less powerful rifle version (the Rail Rifle) used by Tau Pathfinder Teams and Sniper Drone Teams but it is not as reliable as the vehicle-mounted version as it can sometimes kill the user (much like the unreliable Imperial Plasma Gun). However, as of the end of the Tau Empire's Third Sphere Expansion in 999.M41, advancements in the technology have recently eliminated the possibility of a misfire in newly-produced Rail Rifles.
Despite recent Tau advances in superconductor technology, Railguns mounted on Tau starships are of such enormous size and require such large amounts of energy to fire that they are grouped into batteries and fired one barrel at a time in sequential order, ensuring the first barrel is loaded and ready before it is charged again. Tau starship Railgun batteries are considered comparable in firepower and effectiveness to the weapons batteries of macrocannons employed by Imperial starships.
The Railgun functions through the manipulation of the electromagnetic force. A magnetically responsive projectile in an electrically conductive sheath is inserted between two conductive rails attached to a power source, making a complete circuit. As the current moves through the circuit it generates a magnetic force that pushes outward, forcing the projectile down the rails. The further the projectile goes out, the greater the current that goes through and the stronger the magnetic force becomes, causing the projectile to accelerate along the full length of the rails, achieving a tremendous velocity by the time it passes the end of the rails and breaks the circuit. After this, another round can be readied as soon as enough charge can be built back up for the next shot.
To achieve a charge necessary to fire a shell at combat velocity typically requires a bulky array of capacitors attached to a powerful generator, which tends to limit the use of the battlefield Railgun to dedicated vehicles or Tau Battlesuits specifically designed to accommodate them. The Rail Rifle gets around this issue with several pre-charged disposable capacitors which are replaced after every shot. While a Railgun is conceptually simple, manufacturing and fielding one presents several engineering challenges, requiring highly conductive materials with strong resistance to thermal expansion, efficient cooling systems, very precise machining processes, and methods of quickly servicing and re-machining the rails in the field to keep their interior surfaces in pristine condition as even minor wear could have a significantly deleterious affect on the weapon's performance.
A Heavy Railgun is a larger version of the standard Railgun and is usually only mounted on the Manta and A-X-10 variant of the Tiger Shark aircraft. Heavy Railguns fire larger rounds at higher speeds, and the projectiles are fin-stabilised to provide extra lift, and therefore extra range, whilst in an atmosphere. The Heavy Railgun's submunition shell is equipped with a drone processor that is programmed to direct the shell accurately towards its target. This capability is mostly used during void combat to find weak points in the armour of enemy starships as standard submunition rounds lack the necessary penetrating power. They are also useful in ground combat, however, as the shell can saturate a extremely large area with its submunitions and deny cover more effectively to enemy forces due to being fired from the air.
- Codex: Tau (3rd Edition), pg. 17
- Codex: Tau Empire (4th Edition), pg. 30
- Courage and Honour (Novel) by Graham McNeill
- Imperial Armour Volume Three - The Taros Campaign, pp. 175, 284
- Imperial Armour Volume Three - The Taros Campaign, pp. 68, 198, 208
- Battlefleet Gothic Armada (Specialty Game Background Book), pg. 98
- Deathwatch: Mark of the Xenos (RPG), pg. 6